Many neurons in and beyond the striate cortex are known to be selectively sensitive to elongated light stimuli at one preferred orientation. We wish to investigate the spatial selectivity of such neurons in the striate cortex and in areas 18, 19, and the Clare-Bishop area of the cat. Our main goal is to determine whether these cells are selectively tuned to the width of single slit stimuli, to a selective spatial frequency of the stimulus pattern, or to some unknown aspect of spatial selectivity. In carrying out these studies one major task is to plot the magnitude of single cell responses as a function of varying the width of stationary and slowly moving slits at the preferred orientation. Additional studies are carried out using sine-wave gratings at different spatial frequencies and using two narrow slits with variable spacings to probe excitatory and inhibitory interactions within the receptive field.